Question

I’m working on optimizing an application . I found that i need to optimize an inner loop for improved performance.
rgiFilter is a 16 bit arrary.

for (i = 0; i < iLen; i++) {
    iPredErr = (I32)*rgiResidue;
    rgiFilter = rgiFilterBuf;
    rgiPrevVal = rgiPrevValRdBuf + iRecent;
    rgiUpdate = rgiUpdateRdBuf + iRecent;

    iPred = iScalingOffset;

    for (j = 0; j < iOrder_Div_8; j++) {


                 iPred += (I32) rgiFilter[0] * rgiPrevVal[0]; 
                 rgiFilter[0] += rgiUpdate[0];

                 iPred += (I32) rgiFilter[1] * rgiPrevVal[1]; 
                 rgiFilter[1] += rgiUpdate[1];

                 iPred += (I32) rgiFilter[2] * rgiPrevVal[2]; 
                 rgiFilter[2] += rgiUpdate[2];

                 iPred += (I32) rgiFilter[3] * rgiPrevVal[3]; 
                 rgiFilter[3] += rgiUpdate[3];

                 iPred += (I32) rgiFilter[4] * rgiPrevVal[4]; 
                 rgiFilter[4] += rgiUpdate[4];

                 iPred += (I32) rgiFilter[5] * rgiPrevVal[5]; 
                 rgiFilter[5] += rgiUpdate[5];

                 iPred += (I32) rgiFilter[6] * rgiPrevVal[6]; 
                 rgiFilter[6] += rgiUpdate[6];

                 iPred += (I32) rgiFilter[7] * rgiPrevVal[7]; 
                 rgiFilter[7] += rgiUpdate[7];

                    rgiFilter += 8;
        rgiPrevVal += 8;
                    rgiUpdate += 8;



}

ode here

Answer 1

Your only bet is to do more than one operation at a time, and that means one of these 3 options:

1. SSE instructions (SIMD). You process multiple memory locations with a single instructions
2. Multi-threading (MIMD). This works best if you have more than 1 cpu core. Split your array into multiple, similarly sized strips that are independant of eachother (dependency will increase this option’s complexity a lot, to the point of being slower than sequentially calculating everything if you need a lot of locks). Note that the array has to be big enough to offset the extra context switching and synchronization overhead (it’s pretty small, but not negligeable). Best for 4 cores or more.
3. Both at once. If your array is really big, you could gain a lot by combining both.